1. Field of the Invention
The invention relates generally to linear polymers that can be cured to form high temperature elastomers and plastics, and in particular to a high temperature elastomer made by curing linear poly(silarylene-siloxane-acetylene).
2. Description of the Related Art
The aerospace industry has a continuing demand for high performance materials that can withstand extreme variations of temperatures. In particular, there is a need for materials that have elastomeric properties, that have thermal, thermo-oxidative and hydrolytic stability at high temperatures (as high as 300-350xc2x0 C.) and that maintain their flexibility below ambient temperatures. For example, fuel tanks of high flying airplanes and space vehicles require sealants that maintain elasticity for up to 10,000 hours of use at temperatures that range from xe2x88x9260xc2x0 C. to 400xc2x0 C. Further, the material must resist swelling when coming into contact with jet fuel and must have excellent adhesion to and inertness toward metallic substrates.
Linear polymers and crosslinked polymers that have repeating units made up of diacetylene groups and siloxane groups are disclosed in, for example, U.S. Pat. No. 5,563,181 to Keller et al and U.S. Pat. No. 5,874,514 to Keller et al, both incorporated herein by reference.
Linear polymers and crosslinked polymers and copolymers made up of silarylene and siloxane units are disclosed in, for example, U.S. Pat. No. 5,578,380 to Babu, U.S. SIR No. H1612 to Rhein et al, and U.S. Pat. No. 5,346,980 to Babu, all incorporated herein by reference.
It is therefore an object of the present invention to provide a linear polymer can be crosslinked to form a polymer that has elastomeric properties, that is thermally and oxidatively stable at high temperatures and that maintains its elastomeric properties at low temperatures.
Another object of the present invention is to provide a crosslinked polymer that has elastomeric properties.
Another object of the present invention is to provide a crosslinked polymer that is thermally and oxidatively stable at temperatures as high as 300-350xc2x0 C.
Another object of the present invention is to provide a crosslinked polymer that maintains its elastomeric properties at temperatures as low as xe2x88x9250xc2x0 C.
Another object of the present invention is to provide a linear polymer made by a method of synthesis wherein the mechanical properties of the linear polymer and of a crosslinked polymer obtained from curing the linear polymer can be controlled.
These and other objects are obtained by linear polymer that has repeating units represented by the formula 
wherein
(a) n is an integer greater than or equal to 0,
(b) x is an integer greater than or equal to 1, and 
xe2x80x83represents an unconjugated acetylenic group when x is equal to 1 or conjugated acetylenic groups when x is greater than 1;
(c) Ar is an aromatic group, and
(c) R1, R2, R3, R4, R5, R6, R7 and R8 are independently selected from the group consisting of alkyl, aryl, alkylaryl, haloalkyl, haloaryl and mixtures thereof.
The invention is further directed to crosslinked polymers by curing a linear polymer as described above.
In the linear polymers and crosslinked polymers of the present invention, the acetylenic groups in the backbone of the polymer provide for crosslinking in comparison to polymers that only have silarylene-siloxane groups. The aromatic groups in the backbone of the polymer provide for improved thermal stability and rigidity, in comparison to polymers that only have siloxane and acetylene groups.
The following detailed description of the invention is provided to aid those skilled in the art in practicing the present invention. However, the following detailed description of the invention should not be construed to unduly limit the present invention. Variations and modifications in the embodiments discussed may be made by those of ordinary skill in the art without departing from the scope of the present inventive discovery.
The invention relates to a linear inorganic-organic hybrid polymer and a crosslinked polymer derived therefrom. The linear polymer is made up of repeating units represented by the formula 
wherein
(a) n is an integer greater than or equal to 0,
(b) x is an integer greater than or equal to 1, and 
xe2x80x83represents an unconjugated acetylenic group when x is equal to 1 or conjugated acetylenic groups when x is greater than 1;
(c) Ar is an aromatic group, and
(c) R1, R2, R3, R4, R5, R6, R7 and R8 are independently selected from the group consisting of alkyl, aryl, alkylaryl, haloalkyl, haloaryl and mixtures thereof.
Particular values for n and x, and particular choices for the side chains R1, R2, R3, R4, R5, R6, R7, R8, and the aromatic group Ar may be selected according to particular properties desired for the linear polymer and for elastomers and plastics made using the compound. For example, increasing the relative number of silarylene and siloxane units (increasing n) increases the chain flexibility. As discussed in more detail below, the relative amount of silarylene-siloxane units and acetylene units in the repeating unit (as represented by the value of n in the formula) can be controlled by selecting the relative molar amounts of reactants in one of the steps of the synthesis of the polymer. Using larger alkyl groups for the side chains R1, R2, R3, R4, R5, R6, R7, R8 increases the solubility of the linear polymer in organic solvents and increases the hydrophobicity and decreases the thermo-oxidative stability of elastomers and plastics made using the compound. Using aryl groups for the side chains R1, R2, R3, R4, R5, R6, R7, R8 increases the stiffness and slightly increases the thermo-oxidative stability of polymers made using the compound. Using larger aryl linking groups for Ar adds stiffness to elastomers and plastics made from the compound. Linear polymers with larger conjugated acetylenic groups (x greater than 2) are more easily cured, but are more expensive to produce.
In the most preferred embodiment, the acetylenic group is butadiyne (x=2), the aromatic group is phenylene, and all the R groups are methyl. The repeating units of this embodiment may thus be represented by the following formula: 
The linear polymer of the present invention has the advantage of being extremely easy to process and convert into elastomers and plastics since it is, depending on the selection of variables and substituents, either a liquid at room temperature or a low melting solid and is soluble in most organic solvents. The linear polymer is thus well-suited to serve as a thermoset polymeric precursor. The linear polymer may be easily produced by the method exemplified in the following reaction scheme, which illustrates the synthesis of the most preferred embodiment, Compound 1, wherein the acetylenic group is butadiyne (x=2), the aromatic group is phenylene, and all the R groups are methyl. The synthesis may be carried out in a one pot, two step reaction. Hexachlorobutadiene, 2, is reacted with four equivalents of n-butyllithium to get 1,4-dilithio-1,3 butakiyne, 3. dimethylaminochlorodimethylsilane, 4, is added to the solution to get 1,4-bis(dimethylamino-dimethylsilanyl)butadiyne, 5.
Meanwhile, Compound 6 is formed by reacting an excess amount of 1,4-bis(hydroxydimethylsilyl)benzene 7 (weak acid) with bis(dimethylamino)dimethylsilane 8 in refluxing toluene. Despite the relative stability of Sixe2x80x94N bonds, they are readily cleaved by acids and various organic and inorganic electrophiles.
Compound 1, the linear polymer, is produced by reacting compound 5 and compound 6. 
Different linear polymers represented by Compound 1 may be created by varying the relative amount of compound 7 and Compound 8 used in the creation of Compound 6 (thereby changing the value of n, which represents the length of the aromatic disiloxyl/trisiloxy group in compounds 1 and 6). In this manner, linear polymers having different properties and processing parameters tailored to specific needs can be easily created.
The crosslinked polymer is made by thermally curing the linear polymer 1. Typically, the curing is carried out for a sufficient time and at a sufficient temperature to allow at least some of the acetylene groups of the linear polymer react intermolecularly with each other to form a crosslinked network.